In most countries, the part of the spectrum commonly known as the Industry Scientific Medicine or ISM band (in the region of 2.4 GHz) is largely unregulated, meaning that no license is needed to make electromagnetic transmissions in this band.
In this unruly part of the spectrum, frequency hopping spread spectrum systems have been found to have good performance. In these systems, the carrier frequency of a modulated information signal changes or hops periodically to another (or possibly the same) frequency of a set of possible frequencies called the hop set. The hopping sequence is governed by the spreading code. FIG. 9 shows the time/frequency occupancy of an exemplary communication between two nodes of a frequency hopping spread spectrum system. Thus, because the frequency hopping spread spectrum system is continually hopping between parts of the spectrum, the effect of narrow band interference in a particular region is limited.
The present invention is concerned with the operation of a frequency hopping spread spectrum communication system in the presence of a “persistent interferer” such as, for example, a microwave oven or a WLAN network operating at a fixed region of the spectrum.
Persistent interferers present two distinct problems to this kind of system.
(i) “System Performance”
Although the use of a frequency hopping spread spectrum system per se limits the degradation in system performance caused by a persistent interferer, the effect on system performance can be significant, especially in the presence of several persistent interferers.
(ii) “System Compatibility”
WLANs often transmit data in large packets. The presence of a nearby frequency hopping spread spectrum regularly hopping into the region of the spectrum used by the WLAN during the transmission of a large packet can have a devastating effect on the WLAN's performance.